1. Field of the Invention
The present invention relates to a resin-reinforced elastomer, a process for producing the same and a pneumatic tire using the same. More particularly, the present invention pertains to a resin-reinforced elastomer which is excellent in modulus, strength, wear resistance, fatigue resistance and the like and which has a low density; to a process for efficiently producing said resin-reinforced elastomer; and to a pneumatic tire in which said resin-reinforced elastomer having the aforesaid characteristics is used in the tread.
2. Description of the Related Arts
A resin-reinforced elastomer, that is, the composition in which a small amount of a polyolefin is dispersed in a vulcanizable rubber such as natural rubber, polyisoprene rubber, polybutadiene rubber and ethylene-propylene rubber to improve the crack growth resistance, modulus and strength has heretofore been produced in general by a method in which a rubber is blended with a polyolefin such as polyethylene, polypropylene and ethylene-propylene copolymer and then the resultant blend is vulcanized.
For example, Japanese Patent Publication No. 78155/1995 (Hei-7) describes a thermoplastic elastomer composition which provides molded articles such as a car bumper that are minimized in dependence of tensile strength on thickness and in anisotropy and have favorable moldability as well as excellent external-appearance, while maintaining impact resistance.
In addition, Japanese Patent Provisional Publication No. 186606/1995(Hei-7) discloses a pneumatic tire which comprises a rubber composition containing a thermoplastic resin in its tread rubber and which is improved in rolling resistance, while maintaining favorable traction characteristics.
However, the resin-reinforced elastomer obtained by the aforesaid method involves the problems of its having directionality because of the oriented resin, deficiency in strength, wear resistance and fatigue resistance and the like, when being used in a tire member of an automobile. It is desirable that a tread rubber be excellent in strength, wear resistance, cut-through resistance and durability against heat generation, each being well-balanced with one another, and also be improved in extrusion processability. With regard to the above-mentioned requirements, the conventional resin-reinforced elastomer as mentioned above fails to sufficiently satisfy the required characteristics. Thus the present actual-situation is that the development of a resin-reinforced elastomer fully improved in the above-mentioned problems is eagerly desired.
A method in which low density polyethylene is blended to form a rubber composition is known as an effective means for enhancing various performances of a tire. Nevertheless, the blending of the low density polyethylene is unfavorable when used in a rubber composition for a tire which is raised in temperature during running, since it has a low melting point and thus remarkably changes the physical properties of the rubber composition while the temperature is elevated. On the other hand, the use of high density polyethylene in place thereof for the purpose of raising the melting point suffers the disadvantage that the Mooney viscosity of the unvulcani-zed rubber composition is raised, thereby causing extreme difficulty in extrusion processing, as described in U.S. Pat. No. 5,341,863.